Lots of thought put into your project. Not to be negative, but PVC will fail on you at the worst time. UV will weaken the PVC in short order. It is not intended to carry a structural load. Maybe if you paint it, it will reduce the UV effect. Also using grade 40 rather than 20 would give it more strength.

Thanks for your replies regarding the cart and your comments are well taken, especially from experienced kayakers. I have only kayaked / fished with my Outback one time and will be using it at least once per week. Hopefully the cart will hold up. Time will tell.